An interesting demo, one that I feel illustrates wave theory and wave harmonics. When we talk of waves its important to understand what a wave is and why its a different way of transmitting energy”
slowed down 4x.
Many questions arise when viewing demos of this nature.
Often the suggestion is that its faked or the students have been conditioned to react by subtle cues from the teacher.
A reasonable person might ask why? what would be the point?
A better question might be:
How is it done
Why is it done
Is it useful
To answer the first question we must understand what a wave is. What makes a wave and why its different then other modes of energy transport.
When a wave is present in a medium (that is, when there is a disturbance moving through a medium), the individual particles of the medium are only temporarily displaced from their rest position. There is always a force acting upon the particles that restores them to their original position. ”
By medium, we refer to the body. Some key points: The displacement of particles is temporary, the faster or higher rate of vibration the less observed movement will be seen or felt. By allowing the praticales to reattain their equilibrium the force can travel on while the body seems to be at rest or shows very little to no movement that would illicit the reactions shown in the clip…
With the correct phase “timing” the amplitude of wave is added to, this is called constructive interference
If the timing is not correct this is called destructive interference
“the phase difference determines whether they reinforce or weaken each other. Complete cancellation is possible for waves with equal amplitudes.”
“The principle of superposition of waves states that when two or more propagating waves of same type are incident on the same point, the resultant amplitude at that point is equal to the vector sum of the amplitudes of the individual waves. If a crest of a wave meets a crest of another wave of the same frequency at the same point, then the amplitude is the sum of the individual amplitudes—this is constructive interference.
Which as in the clip, elicits many of the reactions due to what is felt, question by those looking at it, as to what is seen.
If a crest of one wave meets a trough of another wave, then the amplitude is equal to the difference in the individual amplitudes—this is known as destructive interference.”